Ink-jet printing is a non-impact printing process in which droplets of ink are deposited on print media, such as paper, transparency film, or textiles. Low cost and high quality of output, combined with relatively noise-free operation, have made ink-jet printers a popular alternative to other types of printers used with computers. Essentially, ink-jet printing involves the ejection of fine droplets of ink onto print media in response to electrical signals generated by a microprocessor.
There are two common means currently available for achieving ink droplet ejection in ink-jet printing: thermally and piezoelectrically. In thermal ink-jet printing, the energy for drop ejection is generated by electrically-heated resistor elements, which heat up rapidly in response to electrical signals from a microprocessor to create a vapor bubble, resulting in the expulsion of ink through orifices associated with the resistor elements. In piezoelectric ink-jet printing, the ink droplets are ejected due to the vibrations of piezoelectric crystals, again, in response to electrical signals generated by the microprocessor. The ejection of ink droplets in a particular order forms alphanumeric characters, area fills, and other patterns on the print medium.
Many inks that are described for use in ink-jet printing are associated with non-thermal ink-jet printing, such as piezoelectric ink-jet printing. Inks suitably employed in such non-thermal applications often cannot be used in thermal ink-jet printing due to the effect of heating on the ink composition.
In commercially-available thermal ink-jet color printers, such as a DeskJet.RTM. printer available from Hewlett-Packard Company, a color spectrum is achieved by combining cyan, magenta, and yellow inks in various proportions. Ink-jet inks are mostly available as dye-based compositions, although a very limited number of black pigment-based ink-jet inks are also available. Titus, cyan, magenta, and yellow inks typically derive their hues from cyan, magenta, and yellow dyes, respectively. The particular set of dyes so employed constitutes a so-called "dye set". Color printers typically employ a four-pen set containing cyan, magenta, and yellow inks as well as a black ink, which is typically used for printing text.
It follows that color thermal ink-jet inks are commonly available as aqueous-based ink compositions that are formulated by dissolving dye in an ink vehicle. For example, a cyan ink would comprise a cyan dye dissolved in an ink vehicle. The dye molecules employed in ink-jet ink compositions are often in the form of dye salts made of a dye anion and a cation such as sodium. These dyes are designed to form solids in the target paper substrate by absorption into paper media by at least two mechanisms. In one mechanism the dye is wicked into the paper and absorbed onto active sites of the paper fiber. There is another mechanism operating in which the ink vehicle evaporates, or is wicked away from the dye, leaving solid dye on and in paper fibers.
Controlling the behavior of printed ink compositions before absorption of the dye salt in the paper media is crucial in attaining good print quality. For example, many thermal ink-jet inks, when printed in various colors on paper substrates, tend to bleed into one another. The term "bleed", as used herein, is defined to be the invasion of one color into another, as evidenced by a visible ragged border therebetween. To achieve superior print quality, it is necessary to have a border between colors that is clean and free from the invasion of one color into the other.
One solution to the problem of color to color bleed between dye-based ink-jet inks involves increasing the penetration rate of the ink into the paper with the use of surfactants. Surfactants lower the surface tension of the ink to increase the penetration rate of the ink into the print medium, thereby reducing the ink's planar spread across and through the print medium into surrounding inks. To effectively control bleed, the surfactant component should be present in the ink above its critical micelle concentration (cmc), as disclosed in U.S. Pat. No. 5,106,416, entitled "Bleed Alleviation Using Zwitterionic Surfactants and Cationic Dyes", issued to John Moffatt et al and assigned to the same assignee as the present application.
Not all surfactants are effective in controlling color bleed between ink-jet inks: rather, there is a large variance in the effectiveness and behavior of surfactants in ink-jet inks. For example, surfactants differ in their solubility in water and, as a consequence, differ in the form they assume in aqueous ink-jet inks. In an aqueous ink-jet ink, surfactants partition themselves between the following three forms: monolayers, soluble form, and micelles. Surfactants that are :substantially soluble in water form hydrophilic monolayers at liquid-solid interfaces such as that between the ink and the metal nozzle plate. Such surfactants are termed "Gibbs monolayer formers". In contrast, surfactants that are substantially insoluble in water exist as micelles in aqueous solution, since they are essentially dispersed therein: hydrophilic monolayer formation by such surfactants is essentially nonexistent. Rather, such water-insoluble surfactants form water-insoluble monolayers that are hydrophobic in nature and are termed "Langmuir monolayer formers".
One measuring stick of whether a surfactant forms Gibbs monolayers or Langmuir monolayers is its hydrophilic-lipophilic balance (HLB). The HLB value empirically quantifies the balance between the hydrophilic and hydrophobic parts of a surfactant molecule in terms of both size and strength. HLB values of nonionic surfactants range from 1 to 40, with lower values indicating greater solubility in oil and higher values indicating greater solubility in water.
Regardless of whether a monolayer is hydrophilic or hydrophobic, monolayers of ink on a nozzle plate can develop into bi-layers of ink. Bi-layers form when a second layer of surfactant molecules lay over the monolayer, organizing themselves into a tail-to-tail arrangement thereupon. Both layers of surfactant molecules in a bi-layer formation have their polar heads external to the layer. The likelihood of a monolayer transforming into a bi-layer depends upon the nature of the surfactant and the nature of the material interfacing with the surfactant. More specifically, the formation of bi-layers results in a film surface more compatible with higher energy surfaces.
The degree of surface energy of a material depends on its attractive forces and is expressed in the same units as surface tension. Examples of materials having relatively high surface energies are gold and nickel; orifice plates in ink-jet printers commonly comprise gold, and possibly nickel. Examples of materials having low surface energies are plastics (such as polyethylene) and lipids.
When an aqueous solution containing no surfactant is applied to a surface having low surface energy, the aqueous solution tends to bead up because the low surface energy of the solid cannot pull against the high surface tension of the liquid. By adding a Gibbs-type surfactant to the aqueous solution, the surfactant forms monolayers at the liquid interfaces, both liquid-air and liquid-solid, with the polar head of the surfactant molecule attracted to the aqueous phase and its lipophilic tail pointing out at the interfaces. Such surfactants lower surface tension by disrupting the hydrogen bonding at the aqueous surface and by providing a lipophilic group to mate with the low energy, solid surface.
In ink-jet printing, however, the orifice plate is typically made of a high surface energy material such as gold, and possibly nickel. Therefore, any monolayers formed by surfactants upon the orifice plate would tend to transform into bi-layers given the high surface energy.
Moreover, it has been determined that, especially among nonionic, ionic, and amphoteric surfactants, those having relatively low HLBs offer the best bleed control between ink-jet inks. Specifically, the above-referenced application Ser. No. 08/501,262, now U.S. Pat. No. 5,536,300, discloses the use of primary surfactants that happen to have relatively low HLB values, namely, secondary alcohol ethoxylates such as Tergitol 15-S-5 and Tergitol 15-S-7, which are available from Union Carbide Co. of Houston, Tex. Tergitol 15-S-5 and Tergitol 15-S-7 have HLB values; of about 10.5 and 12.1, respectively. Secondary alcohol ethoxylates contain (a) an aliphatic chain having a prescribed number of carbon atoms in the chain, and (b) a prescribed number of ethoxylated units. Such ethoxylates are commercially available as mixtures of ethoxylates, and so are described in terms of the predominance of a given compound. For example, "Tergitol 15-S-5" represents a secondary alcohol ethoxylate surfactant predominantly having 15 carbons in its aliphatic chain and 5 ethoxylated units. Secondary alcohol ethoxylates suitably employed in Ser. No. 08/501,262 now U.S. Pat. No. 5,536,300 predominantly have about 12 to 18 carbons atoms in the aliphatic chain, while the number of ethoxylated units is predominantly in the range of 4 to 8, and preferably in the range of 5 to 7 units. It is noted that Tergitol 15-S-5, while having a relatively low HLB value, retains some degree of water solubility. Therefore, Tergitol 15-S-5 is not wholly water-insoluble and is not a true "Langmuir monolayer former". For purposes of discussion herein, the Tergitol surfactants are termed "Langmuir-like" monolayer formers, since they impart poor wetting characteristics to aqueous ink solutions, evidenced in the "beading up" of inks containing Tergitol surfactants on metal orifice plate surfaces.
Examples of other surfactants that are successfully employed to control bleed in ink-jet ink compositions include the class of amine oxide surfactants, such as the following: N, N-dimethyl-N-dodecyl amine oxide (NDAO); N,N-dimethyl-N-tetradecyl amine oxide (NTAO); N,N-dimethyl-N-hexadecyl amine oxide (NHAO); N,N-dimethyl-N-octadecyl amine oxide (NOAO); and N,N-dimethyl-N-(9-octadecenyl) amine oxide (OOAO).
It is noted that HLB values for particular surfactants are typically reported as their hydrophilic-lipophilic balances in water, not an aqueous ink system. While the absolute HLB value for a surfactant in water might be a slightly imprecise valuation of its HLB in an aqueous system with 25 wt % co-solvent, the absolute HLB values are still useful in discussing the solubility nature of the surfactant in an aqueous environment. Moreover, it is contemplated that the imprecision in HLB value when employing a co-solvent in an aqueous solution is slight, as evidenced by only a slight increase in cloud point temperature upon adding a co-solvent, as described in greater detail below.
When a surfactant having a relatively low HLB value is dispersed in an aqueous solution, it tends to exist primarily as micelles in solution. At the liquid-solid interface, such surfactants are Langmuir-like monolayer formers and tend to form hydrophobic monolayers, rather than the hydrophilic monolayers formed by relatively high HLB surfactants. When a Langmuir-like monolayer former is employed in an aqueous ink solution along with a high surface energy orifice plate, it tends to form disorganized and hydrophobic monolayers, and sometimes poorly organized bi-layers of surfactant molecules on the liquid-solid interface, with the more polar ends of the surfactant molecules facing toward the solid interface. Consequently, ink containing a surfactant of low HLB, which tends to form monolayers and bi-layers with some Langmuir-type nature, is poorly wetting on a high energy surface such as gold or nickel orifice plates, and the ink will thus form puddles with a high contact angle on these surfaces, rather than wetting with low contact angle and spreading over the gold orifice plate surface.
Wetting characteristics are indicated by contact angle on a given surface, with contact angle being inversely proportional to the degree of wetting imparted to the ink. The formation of puddles evidences a high contact angle on the metal surface, and hence it is concluded that Langmuir-like monolayer formers impart poor wetting characteristics to aqueous ink solutions. In fact, without subscribing to any particular theory, it is speculated that one reason for the relatively low surface tensions of inks containing Tergitol 15-S-5 is the poor wetting of the metal ring used for surface tension measurements, aside from the surfactant's contemplated behavior of disrupting the attractive hydrogen bonding forces at the surface of the liquid.
The poor wetting characteristics of such low HLB surfactants have been found to adversely affect print quality. More particularly, when ink-jet inks made with low-HLB surfactants such as Tergitol 15-S-5 are fired from a thermal ink-jet print cartridge such as one of Hewlett-Packard's DeskJet.RTM. printers, the ink tends to form puddles having high contact angles on the metal orifice plates. These puddles of ink may consist of a few small drops of ink or may cover major portions of the metal orifice plate. Regardless, such high-contact-angle puddles, positioned near any orifice, can cause deflection of ink-jet ink drops jetted through the orifice to a print medium. In severe cases, high-contact-angle puddles of ink on the orifice plate can completely occlude an ink-jet orifice. Thus, while surfactants having low HLB values may be effective in controlling bleed in dye-based ink-jet inks by lowering surface tension, these same surfactants do not provide adequate wetting characteristics to the ink.
An additional problem associated with the use of ethoxylated primary surfactants such as Tergitol surfactants is a reduction in ink temperature cloud point. The cloud point of an ink is that temperature at which the primary surfactant comes out of solution, thereby clouding the visible appearance of the ink, which is an undesirable occurrence. In general, the solubility of surfactants increases with temperature. However, the solubility of these ethoxylated primary surfactants decreases as the temperature is raised, such that their solubility is inversely proportional to the temperature of the ink. It follows that an ink containing a surfactant having a low HLB value which also possesses a polyethyleneoxide group at its solubilizing or polar head has a low temperature cloud point above which the surfactant is insoluble. For example, Tergitol 15-S-5 has such a polyethyleneoxide group and exhibits a temperature cloud point of 29.degree. C., above which temperature the surfactant becomes insoluble.
Tergitol 15-S-5 is the most problematic of the Tergitol surfactants with regard to ink temperature cloud point, but Tergitol 15-S-7 also exhibits this problem to a certain extent. As the number of ethoxylated units increases, the cloud point likewise increases to a more desirable temperature, which is expected given the increased solubility deriving from an increased number of ethoxylated units. It is noted that increasing the concentration of an ethoxylated surfactant in an ink-jet ink increases its solubility in the ink and, consequently, also increases the cloud point temperature.
Accordingly, a need exists for an ink-jet ink composition and method of printing that exhibits improved wetting characteristics and an increased cloud point without sacrificing the bleed control achieved with the use of surfactants having low HLB values.